Bilirubin, a catabolic product of red cells and hemoproteins, is normally detoxified by conjugation with glucuronic acid in liver through the glucuronyl transferase system. It is an extremely toxic compound to some tissue cells and so it is bound with serum albumin to prevent its entry into tissues--other than the liver--to where it is transported via the vascular stream. However, the hepatic glucuronyl transferase system for the detoxification of bilirubin as well as other parts of the degradative pathway is only slowly developed in neonatal life. Thus, if red cells are broken down at an abnormally high rate the bilirubin accumulates and can rise to heights where it may exceed the albumin bindng capacity and then enter the body cells. The new born brain is especially vulnerable to the deleterious effects some of which include inhibition of respiration, uncoupling of oxidative phosphorylation, abolition of respiratory control, large irreversible swelling of mitochondria, and rapid loss of cellular ATP and viability. Clinically it is extremely difficult to estimate the serum bilirubin concentration at which bilirubin can no longer be totally bound to albumin at which point it will seek tissue sites. This project is designed to determine the following: (a) the mechanism of the cellular toxicity of bilirubin, (b) the factors that govern the binding of bilirubin to lipids and protein, (c) a method that can detect the point at wich serum albumin is saturated with bilirubin, and (d) how to apply the information from (a), (b), and (c) clinically to the hyperbilirubinemic newborn so that no newborn will suffer brain damage from this bile pigment. A variety of methods will be used to evaluate these points including a new technique we have devised to measure free and bound bilirubin in serum. Artificial membrane systems will be a major tool to determine the toxicity mechanism.